Page 25 - MetalForming February 2017

Among Osborn’s recommended practices for using coil die springs: Follow recommend- ed procedures for guiding. “Guiding on the spring ID (center illustration) is preferred,” Osborne says, “even though in most applications stampers wind up guiding on the OD (left). But by investing a little more upfront to guide on the ID, you create less contact area and minimize wear.
“Typically a metalformer can get any two of these three,” he says. “And with coil springs it’s the same—where the three variables are force, deflection and spring life, and you can only get two of the three. For example, if you need a lot of force over a large deflec- tion, don't expect long life. In this case, nitrogen-gas springs might make sense.”
An exception to this rule, says Osborne: springs used for misfeed protection within, say, an in-die tap- ping device. “You need high force and long travel,” he says, “and you don’t care about life because the springs operate statically—in a healthy appli- cation they never cycle. So, even though they are under considerable preload and expected to travel a long distance, they rarely if ever actuate, so their life will not be a consideration.”
Lastly, Osborne issues a warning to manufacturers looking to use die springs for non-die or mold applica- tions, which he says is becoming more common. Here the user needs to fully understand spring rate, or the spring constant, that governs the force-deflec- tion curve.
“Typically, spring manufac- turers provide a spring-rate tolerance of ±10 percent,” Osborne says. “This works fine for dies and molds, but in non-tooling applications it may not suffice; the application might require a tighter tolerance on spring rate. For these appli- cations, many manufactur- ers offer (for a premium) springs with the constant controlled to within ±3 to 5
percent.”
In gas-spring applications requiring a lot of die lube, be sure to protect the springs from contamination by using protective rod covers (shown here) or wipers. “Contamina- tion is a leading cause of spring failure,” says Special
Springs’ Jim Glynn.
list of best practices:
• Always incorporate preload (a min-
imum of 5 percent of free length), achieved by correctly specifying the spool/keeper lengths used in securing the stripper pad, lifter rail or other moving element being considered.
• Ensure flatness (to within 3 percent) and perpendicularity of the contacting surfaces, to within 1 deg. or less.
• Follow recommended guiding prac- tices. “Guiding on the spring ID is pre- ferred when possible,” Osborne says, “even though in most applications stam- pers wind up guiding on the OD. But by investing a little more upfront to guide on the ID, you create less potential contact area and minimize wear.”
• Reduce or eliminate the possibility of scrap material or other foreign objects becoming trapped between the coils, by adding spring cans or other guarding to the setup.
• Never cut springs to length. Always purchase and install the correct spring for the application. “Cobbling springs to force-fit an application is penny- wise and pound-foolish," says Osborne.
• Replace broken springs immedi- ately, and always replace springs as a complete set.
• Be mindful of increases in deflec-
tion due to shut-height changes that result from routine die sharpening.
• Be mindful of series versus parallel arrangements. The resulting load cal- culations are very different.
• When inspecting springs to gauge their condition, look for worn paint or shiny surfaces, indicators that surfaces are rubbing. “Sur-
faces should be free of stri-
ations or cracks,” Osborne
says. “Visually inspect
them every time the die
goes in for sharpening or
general maintenance. And
any time the stripper pad
is removed and the springs
can readily be removed,
give them a rattle—if
they’re cracked you will
hear or feel it.”
Spring Selection— Shoot for Two
out of Three
When selecting the right coil spring for the job at hand, Osborne offers a sim- ple and often-cited anec- dote, along the lines of ‘good quality, low price and fast delivery.’
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